Councils are already facing a wave of legal action over cuts to funding for pupils with SEND, which covers everything from dyslexia and learning difficulties to physical disabilities, autism and complex needs.

Chief secretary for the treasury, Liz Truss, reportedly pushed for an extra £155 million to fund special needs places in the recent budget so local authorities could avoid using private providers, but was rebuffed by the chancellor.

Yet that would still only cover a fraction of the £565 million local authorities spent on sending 10,520 SEND pupils to private special schools last year, according to the data compiled by Tes.

The biggest increase came in Kent, where spending on sending pupils to special schools almost doubled from £15 million on 340 pupils in 2012/13 to £28.5 million on 680 pupils in 2017/18.

In Birmingham, the £9.46 million the city council spent sending 127 pupils in private special schools in 2012/13 ballooned to £20.72 million on 709 students.

Norfolk increased spending from £11.29 million on 483 pupils to £20.47 million on 722 pupils, while Hampshire also saw £8.35 million of funding for 239 students rise to £19.38 million for 485 students.

Stripping out councils which did not provide comparable data, spending rose 40 per cent over the period while the number of pupils in special schools increased by 42 per cent.

University of Exeter education specialist Alison Black said there has been a growing “trend towards segregation” in British education, with the number of primary-aged pupils sent to specialist schools rising particularly fast.

"There is nothing that happens in a special school that can’t happen in mainstream education," said Dr Artemi Sakellariadis, director of the Centre for Studies on Inclusive Education (CSIE).

"If nothing is being done by central government and little is being done by local authorities to increase the capacity of ordinary schools to cater for the full spectrum of learning, then demand for special schools will increase."

Much of the shift has been driven by dwindling mainstream school budgets, which the Institute for Fiscal Studies calculates have fallen 8 per cent per pupil since the Conservatives came to power in 2010.

That has made it harder for local authorities to fund the 30,000 SEN pupils with a statement or Education Health and Care (EHC) plan who entered the education system over the same period requiring extra support.

Council bosses have warned schools are being pushed to the brink by a funding gap of more than half a billion pounds for children with special educational needs and disabilities.

A recent survey by the National Association of Headteachers found 94 per cent of heads said it is now harder to resource support for SEND pupils than two years ago.

Experts also pointed to several other more complex drivers behind the growing numbers of SEN pupils being sent to independent special schools:

The focus on exams in the curriculum and the resulting pressure on teachers to produce good results

The increasing number of academies, which it has been claimed are less keen on taking children with SEND

The growing number of children with complex special needs that cannot be met by mainstream schools

The Children and Families Act 2014, which has given parents a greater say in where their children go to school

Claire Dorer, chief National Association of Independent Schools and Non-Maintained Special Schools (NASS), said most pupils taught by her members are there because they have no other choice.

“Parents who have had their child identified as having particularly complex needs often want a specialist placement,” she said. “If every other school is full, where are you going to place children?”

A Department of Education spokesperson said: “Our ambition for children with SEND is exactly the same for every other child – to achieve well in education, and go on to live happy and fulfilled lives.

"This is why we’ve introduced Education Health and Care plans, tailored to each individual, to ensure they are supported while they study.

“Local authorities and schools have statutory duties to support children and young people with SEND.

"In 2018-19 councils will receive almost £6 billion of funding for young people with more complex SEND – an increase from £5 billion in 2013.

"However, we recognise that local authorities are facing cost pressures on high needs, which is why we are monitoring local authority spending decisions and keeping the overall level of funding under review.

“We recognise that independent schools play an important role in the schools system.

"Parents are best placed to make the right choices for their children that cater for their needs.”

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